6 ELEMENTS OF PALAEONTOLOGY 



called fossil organism, JEozoon, occurring in gneiss, has been proved to be of 

 inorganic nature. 



The Palaeozoic or Primary Group comprises the Cambrian. Ordovician, 

 Silurian, Devonian, Carboniferous, and Permian systems, each of which is 

 made up of a great number of series, stages, and zones. In the Cambrian 

 crustaceans (trilobites), mollusks and worms predominate, associated with a 

 few Pelmatozoa, coelenterates, sponges, and poorly preserved algae. In the 

 Silurian system all classes of the animal kingdom are represented with the 

 exception of amphibians, reptiles, birds, and mammals, while the flora still 

 consists of algae. Marine invertebrates are very abundant, especially crus- 

 taceans, mollusks, echinoderms, and coelenterates, while only a few frag- 

 mentary fish-remains indicate the presence of vertebrates. All the species, 

 and nearly all the genera, have since become extinct, and belong for the most 

 part to extinct families and orders. During the Devonian, Carboniferous, and 

 Permian systems, the same classes of animals continue as a body, but are 

 represented by totally different families and genera. Fishes develop a great, 

 variety of forms in the Devonian, amphibians (Stegocephalia) make their 

 appearance in the Carboniferous, and reptiles in the Permian. The flora con- 

 sists chiefly of vascular cryptogams, together with a few conifers and cycads. 



The Mesozoic Group comprises three systems the Triassic, Jurassic, and 

 Cretaceous. Many of the widely distributed Palaeozoic types (Tetracoralla, 

 Graptolites, Crinoids, Cystids, Blastoids, Brachiopods, Trilobites) have either wholly- 

 or in greater part disappeared, while others (Cephalopods, Lamellibranchs, Sea- 

 urchins) are replaced by very different genera and families. Vertebrates are 

 remarkable for the gigantic size attained by amphibians (Labyrinthodonta) and 

 many reptiles, as well as for the wonderful variety of the latter. Birds appear 

 for the first time in the Upper Jurassic (Arcliaeopteryx), and mammals towards 

 the close of the Triassic, being represented by diminutive, probably mar- 

 supial types. During the Triassic and Jurassic periods, vascular cryptogams, 

 conifers, and cycads remain the dominant plant forms, dicotyledons not occur- 

 ring until the middle Cretaceous. 



The Cenozoic Group comprises the Tertiary and Post-Tertiary or Quaternary 

 systems. Among the invertebrates, ammonites, belemnites, Rudistae, and 

 most of the Crinoidea have now passed away. Amphibians and reptiles have 

 greatly declined, and, like the invertebrates, -are represented by still living 

 orders. On the other hand, birds, and most particularly mammals, attain a 

 wide distribution ; the latter class branches out in such manifold variety, and 

 experiences such rapid development during Cenozoic time, that it alone 

 furnishes us with the principal index-fossils of this era. From now on the 

 flora consists chiefly of dicotyledonous plants. 



Palaeontology and Physical Geography. Not only do fossils con- 

 stitute the very foundation of historical geology, but they furnish -us in 

 addition with invaluable information respecting the origin of the rocks in 

 which they occur, the former distribution of land and water, climatal 

 conditions, and the laws of geographical distribution that have prevailed in 

 former periods. By means of analogy with recent species we are able in 

 most cases readily to determine whether fossil forms pertain to land, fresh, 

 brackish, or salt water species, whence it is apparent under what conditions 

 the strata were deposited. The distribution of marine and fresh-water 

 formations helps us to certain conclusions respecting the extent of former 



